Global navigation satellite systems (GNSS) exist in which information received by line of sight from multiple satellites orbiting the earth is used to determine the position of a receiver of the information. Several GNSS systems exist, each with their own satellites and transmission specifications. Examples of these systems may be, for example, the global positioning system (GPS), GLONASS, BeiDou Navigation satellite system and the European Galileo, some of which may be in the earlier stages of implementation.
Very generally, a GNSS receiver works by calculating the receiver position based on received satellite signals. Each satellite transmits an indication of its position and time of transmission. On reception, the receiver can determine the time taken for the transmitted signal to reach the receiver based on the time of transmission indicated in the signal and highly accurate timing at the receiver. This time taken can be translated into a distance from the satellite to the receiver. On reception of such signals from four or more satellites, the receiver can calculate its position in three dimensions.
In particular, the satellite signals can comprise a repeating pseudo random code that is unique to each satellite. During a tracking phase of the receiver, the pseudo random code from the satellite is correlated against an identical repeating pseudo random code at the receiver. The offset between the codes indicates the time difference between the transmission of the signal and the reception of the signal. The position of the receiver relative to the satellite may be determined from this time difference. By combining the receiver positions relative to multiple satellites, the receiver may determine its absolute position.
The receiver must first undergo an acquisition phase with the satellites in order to synchronise the pseudo random codes at the receiver to the pseudo random code at each satellite. During this acquisition phase, the receiver may also receive almanac and ephemeris data regarding the satellites.
The almanac data is an indication of the orbital course for each satellite in the GNSS system. The receiver can use this to determine which satellites should be visible to the receiver and hence which satellite signals to attempt to acquire and track. The almanac data comprises information about the status of the satellites and approximate orbital information. In a GPS system, it takes about 15 minutes to receive the almanac and the information in it is considered valid for up to 180 days.
The ephemeris data indicates the positions of the satellites at any instant for a period of time. During the tracking phase, the receiver may use this ephemeris data to identify the position of each satellite at any given time. The ephemeris data contains precise information about the orbit of that satellite. In GPS, the ephemeris may be updated every 2 hours and is usually valid for 4 hours.
Valid almanac data, ephemeris data, initial location of the receiver and an initial time at the receiver may be required by a GNSS receiver in some states in order to acquire a position. In GPS, the initial location should be accurate within 100 km and may include an initial velocity within 25 m/s accuracy. The initial time should be accurate within 20 seconds.
The time taken from start-up to being able to provide a position of a receiver may differ depending on what information is already available at the receiver, in other words the state of the receiver. This period can be called the time to first fix (TTFF) and can vary between 5 seconds to an hour depending on the initial state of the receiver. The receiver may be in three states—cold (factory) start; warm (normal) start; and a hot (standby) start.
In a cold or factory start, the receiver has none of the above data available and must acquire both the ephemeris and almanac data as well as initial position and time. In this state, the TTFF may be between 15 minutes and an hour.
In a warm or normal start, the current almanac, initial position, and time information at the receiver may all be valid. In this state, the receiver acquires the ephemeris data before a fix can be obtained. The TTFF may be 30 seconds to 2 minutes depending on satellite availability and the type of GPS receiver.
In a hot or standby start, the time, position, almanac, and ephemeris data are all available at the receiver. This may enable a rapid acquisition of satellite signals. The time required for a receiver in this state to calculate a position fix may also be termed time to subsequent fix (TTSF). If the receiver has been off for less than an hour, TTFF (or TTSF) may be in the region of 5-20 seconds.
Some navigation applications may be more efficient if the TTFF can be kept as short as possible. Embodiments of the present application aim to address such concerns.